It is well know in the art that many cross-linkable polymeric systems can be arrested at a prepolymer stage to provide a curable coating composition that can be fully cross-linked at a later stage. These are often referred to as “B-stage” polymers. There are many organic, inorganic, and organo-metallic polymers that fit this description. Examples of organic prepolymers are polyepoxides polyurethanes, and precursors to fully condensed polymers such as polyamic acids that can be converted to polyimides. Examples of organo-metallic prepolymers are polysiloxanes and titanates. Some of these B-stage prepolymers are used as tie coats for improving adhesion of additional layers of coatings. Other prepolymers are often internally formulated so as to arrive at the final cured coating properties without any attempt to modify the cured properties by topically applying other reactants to form a gradient structure.
It is often desirable to apply a protective coating to items made of plastic, glass, metal, and other materials. These protective coatings impart certain desirable characteristics to the material including, but not limited to, scratch resistance, mar resistance, wear resistance, static resistance, and corrosion resistance. The coatings can also be used as a release agent. It is also desirable that the coated material maintain adequate resistance to common chemicals, thermal and mechanical cycling and shock, and humidity. The ease of formulating and applying the coating are also important characteristics.
Many articles traditionally made of glass are increasingly being manufactured from various plastic materials due to their light weight, low cost, and ease of manufacture. Specifically, polycarbonate and CR-39 (diethyleneglycol bis allylcarbonate) are often chosen as glass substitutes for optical products such as ophthalmic lenses for prescription eyewear and safety glasses, face shields, aircraft windows, picture frames, mirrors, and the like. Polyacrylate is often chosen for its optical clarity and heat resistance. The cases of electronic products, such as computers and televisions, are often made of polystyrene, as are other clear, rigid containers that need to be economical. Disposable syringes and other medical devices can be made of polypropylene. Polyester and polyacetate are used for photographic film substrates, and polyester is also used for liquid crystal displays. Polyacetal is used for applications requiring dimensional stability. It is desirable that all of these materials exhibit scratch, mar, and wear resistance to maintain optical clarity and a mar-free appearance.
Protective eyewear based on polycarbonate (e.g., eyeglasses, goggles, face shields) is ubiquitous. It is found in laboratories, chemical plants, machine and woodworking shops, sports equipment, and dental offices to name a few. Polycarbonate is one of the world's foremost engineering materials and its usage in protective eyewear is based on its optical clarity, impact resistance, toughness, light weight, high refractive index, and excellent weatherability due to added ultraviolet (UV) light stabilizers. However, its surface is comparatively soft and is prone to wear and scratching even with a tissue. Despite widespread required use in the workplace, there has been a significant reluctance among many workers to wear safety eyewear. Usual complaints include the inability to see clearly and comfortably because the eyewear is scratched. Because most companies provide protective eyewear to their employees, replacement eyewear costs can be substantial.
In order to attenuate the scratching propensity of plastics, primarily those plastics used in ophthalmic lenses and safety eyewear, the chemical industry has developed a number of hard, abrasion resistant coatings that are generically called “hardcoats.” These coatings do improve the scratch resistance of polycarbonate to some extent. However, the level of protection does not approach the scratch resistance of glass. There is still a need to increase the scratch resistance of these plastic products, as well as products made of other substrates, to extend their useful life.